Natural water typically contains many salts which form ions in solution. All these ions are involved in chemical equilibria. The solubility of the different chemical species is described by the solubility product. If the solubility product of a chemical species is exceeded, solid products can precipitate out of solution and produce deposits on surfaces in contact with the aqueous solution. These deposits can give rise to severe problems in industrial aqueous equipment such as heat exchangers in cooling systems, boilers, evaporators, etc. The most common deposits which form on aqueous equipment surfaces are calcium carbonate, calcium sulfate, calcium phosphate, zinc hydroxide, zinc phosphate, silicates and iron oxides.
To overcome these problems, deposit control chemicals such as phosphonates and polymers have been used to treat these systems. At very low (substoichiometric) treatment levels, these deposit control chemicals inhibit the nucleation and growth of crystals.
For example, U.S. Pat. Nos. 3,904,522 and 3,928,196 describe the use of polyacrylic acid and hydrolyzed polyacrylamide as scale inhibitors for CaCO.sub.3 and CaSO.sub.4. U.S. Pat. No. 3,928,196 describes the use of acrylic acid-2-acrylamido-2-methylpropylsulfonic acid (AMPS) copolymer as an antiprecipitant for calcium carbonate and calcium sulfate. U.S. Pat. No. 3,806,367 teaches the use of acrylic acid-AMPS copolymers in preventing and removing iron oxide deposits.
Prior to about 1980, there were no commercially available polymers or phosphonates which could prevent calcium phosphate, zinc hydroxide and zinc phosphate deposit formation. The copolymer acrylic acid-hydroxy-propylacrylate was first described as preventing these deposits (European Patent Application 0 017 373). Since 1980 many other polymers have been developed which are also able to prevent zinc hydroxide, zinc phosphate and calcium phosphate deposits. For example, European Patent Application 0 142 929 and U.S. Pat. No. 4,288,327 describe the use of acrylic acid-sulfonate ether copolymers as phosphate antiprecipitants.
The most widespread phosphonates are hydroxyethylidene phosphonic acid and nitrilotrimethylenephosphonic acid. Phosphonates are also used as corrosion inhibitors. However, the use of phosphonates as scale and corrosion inhibitors in water treatment has left some important problems. As is known from the literature, many of the commonly used phosphonates form insoluble phosphonate salts with cations, such as, for example, calcium and zinc.
Until now only sulfonate containing polymers were known to prevent calcium and zinc phosphonate deposits. For example, European Patent Application 0 142 929 and European Patent Application 0 122 013 describe the use of sulfonate containing polymers for this purpose.
Now, surprisingly, it has been found that sulfate containing polymers also exhibit zinc and calcium phosphonate stabilizing properties. Sulfate containing polymers have been known for a number of years. For example, U.S. Pat. No. 3,839,393 describes the synthesis and polymerization of sulfatoalkane acrylates and methacrylates. U.S. Pat. No. 4,036,788 describes anionic hydrogels based on sulfate containing monomers. However, in the literature phosphonate stabilizing properties of sulfate containing polymers have never been described, suggested or recognized. While U.S. Pat. No. 4,036,788 to Steckler is involved with the complexation of basic cationic materials onto anionic hydrogels which are stable, three dimensional, water-insoluble copolymer networks, the present invention is directed to water treatment with substoechiometric amounts of the water-soluble polymers disclosed.